A database has been used in a variety of applications and a Relational Database (“RDB”), among others, has been mainly used in a medium-scale or large-scale system, because the RDB is capable of removing logical inconsistencies. For example, the RDB is used in an airline reservation system and the like. In this case, reservation targets (or mostly one target) to be identified by the system can be rapidly retrieved, or the system can confirm, cancel, or change a reservation. Furthermore, the number of vacant seats on a particular flight can be determined because the number of seats on each flight is no more than at several hundreds at most.
It is known that such RDB is suitable for handling tabular data, whereas the RDB is not suitable for handling the tree data (e.g., see non-patent document No. 1).
Furthermore, some of the applications are adapted to be represented not by the tabular data but by the tree data. In particular, XML (eXtended Markup Language) adopting the tree data structure as a data standard for Internet or Intranet applications has recently spread (e.g., see non-patent document No. 2 to know details about XML).
However, handling of the tree data structure, including retrieval of the tree data, is generally very inefficient. The first reason for this inefficiency is that it is difficult to locate quickly where the data should exist, as it is contained in many nodes in various places. In the RDB, for example, the data regarding “age” is stored only in the field “age”. In the tree data structure, however, since the nodes containing the data regarding “age” are located all over the place, in general, it is not possible to retrieve relevant data unless we search through the whole tree data structure.
The second reason for this inefficiency is that it takes time to represent a result of the retrieval. In the case of the tree data structure, attempting to represent a node group whose nodes meet retrieval requirements often needs to represent descendant nodes of those nodes and it takes time to represent the descendant nodes since the data structure for the tree is ad hoc unlike a RDBMS (Relational DataBase Management system).
Therefore, conventional methods have been proposed for converting tree type data into the RDB in order to take advantage of the RDB which is a main stream of the database when it is required to convert the tree data structure into the database (e.g., see patent document No. 1). In the RDB, the data is decomposed into a table (tabular form) to be stored therein. To this end, the tree type data has to be packed into the table in order to convert the actual tree type data into the RDB. However, it is required to individually pack the data into the table and design a system depending on the data structure. Therefore, building the system based on the RDB is a very troublesome task.
In addition, a method has been proposed for converting a tree type data, in particular an XML data, into the database while keeping its data structure. In a tree type data structure, since descendant nodes are created from one node and a variety of descriptions are allowed to describe the structure, the troublesome task of designing the system can be remarkably alleviated. Accordingly, there is an increased need to treat tree structure data using a technique for handling the tree structure like the XML as a core technology.
One exemplary approach of converting XML data into the database, while keeping its data structure, consists of acquiring a copy of the data written into the tree structure and separately holding index data for retrieving the data. An example of this is the index data in terms of “age” if the field of “age” is concerned (e.g., see patent document No. 2). This provides for the ability to not only take full advantage of the XML data, in that an attribute can be added to the data itself, but also to store a relational structure of each field described by a tag.    Patent Document No. 1: JP2003-248615A    Patent Document No. 2: JP2001-195406A    Non-patent Document No. 1: SEC Co., Ltd., “Karearea White Paper”, [online], [searched on Feb. 19, 2004], Internet URL:http://wwww.sec.co.jp/products/karearea/    Non-patent Document No. 2: W3C, “Extensible Markup Language (XML)) 1.0 (Third Edition)”, [online], Feb. 4, 2004, [searched on Feb. 19, 2004], Internet <URL:http://www.w3.org/TR/2004/REC-xml-20040204/>